media: via-camera: use struct v4l2_fh

Modern V4L2 drivers should use struct v4l2_fh to represent a filehandle.
This driver was one of the few that didn't use it.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reviewed-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>